Concertina-like folding doors include lifting straps fixed to the bottom bar and passing through rings fixed to at least some of the other bars. All such doors include drive means having a winding shaft disposed above the curtain and onto which the curtain itself or the straps are wound. The edges of the curtain and/or the ends of the reinforcing bars are capable of moving in slideways integral within or by lateral uprights which support a top horizontal cross-member on which the winding shaft is mounted, possibly together with an electric motor and control means for the motor.
In order to be able to perform their functions correctly, the reinforcing bars must have a degree of stiffness and a weight sufficient to carry and support the curtain during winding and unwinding operations. Unfortunately, e.g. due to clumsy operation, it may happen that the curtain is lowered too quickly and falls down onto a person or a vehicle that has not cleared the doorway. This can give rise to personal injury or to damage to vehicles and/or to the door itself. An object of the present invention is to avoid such drawbacks.